Step 24: Update, V0.3

Step 25: Local NFB Option, V0.4

Late Sept., 2008While reading about the weird additions to the Fender Bassman (Ver AB165), I noticed a comment about a local Negative FeedBack loop on...

Step 3: The Tubes

Availability

If we still wanted 50L6 tubes, they are fairly plentiful--lots of radios used these tubes. Same with 12AX7's, they're still being manufactured today and are plentiful (although not cheap.) I already had three for my Ampeg...

But choosing the 6DG6GT for a power tube was truly a pleasant surprise. The tube was standard in many TV sets, and they're cheap and easy to find. I bought 4 from ebay, at a cost of only $3.50 each! (shipping included!) Contrast that with 6V6's--good ones run $20+ per tube, minimum...

And availability is always a concern. No point in building an amp if you can't buy any replacement tubes.

(Pardon the weird formatting--Instructables yanked the ability to use PRE tags, and screwed up the conversion when they did so. I've tried to fix it the best I can...)

But if the 6DG6GT is to have the same electrical characteristics as the 50L6, the heaters must perform in a nearly identical fashion. The filament wire itself must be designed to compensate -- using more current, at the lower voltage:

Volts X Amps == Power consumption

50L6 : 50V * .15A = 7.5 watts
6DG6GT : 6.3V * 1.2A = 7.56 watts

Obviously, the heater power requirements are practically identical; of course that follows since the electrical characteristics of the tube also match. But 2.4 amps (two 1.2A tubes, and not counting the preamp tube) is a fairly high amount of heater current @ 6V for a small amp... (the total heater current is 2.7A @ 6.3V)

(You should look at simple push-pull amps like the Fender Super 5D4 instead, if you want to use 6L6 tubes. A push-pull output transformer for a pair of 6L6's will be cheaper than a single-ended transformer for a pair...)